1. Field of the Invention
The present invention relates to induction motor rotors and rotors therefor, and, more particularly, to induction motors having a cage rotor.
2. Description of Related Art
Conventionally, cage rotors are employed in induction motors, as described in, for example, Japanese Laid-Open Utility Model Publication No. 5-50981. A cage rotor includes a substantially cylindrical iron core, a plurality of aluminum conducting members, and a pair of aluminum end rings. The core is formed by laminating a plurality of disk-like core sheets, each of which is configured by a silicon steel sheet. A plurality of axial through holes are formed in a peripheral portion of the core. Each of the aluminum conducting bars is fitted in a corresponding one of the through holes. The end rings bind the corresponding ends of the aluminum conducting bars together, thus forming a cage shape.
If the induction motor including the cage rotor is used for, for example, a turbocharger as in Japanese Laid-Open Patent Publication No. 1-122351, the induction motor must tolerate high-speed rotation, which may reach 100,000 rpm. More specifically, in this case, relatively large centrifugal force acts on the cage rotor. Therefore, relatively large tensile force acts on each of the end rings in a radial outward direction. It is thus required that the end rings have relatively high rigidity.
However, conventionally, the end ring of the cage rotor is formed of aluminum casting material (aluminum die-cast material). Therefore, the end rings do not have the required level of rigidity for tolerating the high-speed rotation. The end rings are thus damaged or deformed, hampering the rotation of the induction motor.
To solve this problem, the aforementioned utility model, includes a pair of end plates formed of stainless steel, which presents sufficiently higher rigidity than that of the aluminum casting material. The end plates are attached to opposing ends of the iron core, such that the ends of the aluminum conducting bars are arranged in the corresponding end plate. In this state, aluminum is cast in the end plates for forming the end rings, which are encompassed by the end plates. The end plates thus support the end rings, preventing the end rings from being damaged or deformed.
Further, in order to dispose the aluminum conducting bars are at radial outward positions in the core, the end rings connecting the ends of the aluminum conducting bars must be enlarged in a radial outward direction. Since the end rings are encompassed by the end plates, the radial outward portion of each end plate with respect to the associated end ring thus has a relatively small radial dimension. Therefore, in order to increase the rigidity of the same portions of the end plates, the end plates are enlarged in the axial direction, as described in the aforementioned utility model publication.
Further, as described in this publication, a portion of the outer peripheral surface of each end of the conducting bars is placed in contact with the associated end plate, such that the radial dimension of each end plate is sufficiently large and the conducting bars are allowed to be placed at the radial outward positions in the iron core. In this structure, the contact area between the outer peripheral surface of each conducting bar and the associated end ring is relatively small. This increases electric resistance between the conducting bars and the end ring. To solve this problem, the end rings are connected to the end surfaces of the conducting bars. The end rings thus become relatively large in the axial direction, making it further necessary to enlarge the end plates in the axial direction.
Since the axially large end rings are relatively heavy, the total weight of the rotor is increased. Therefore, if the rotor is supported by a rotary shaft in a cantilever manner, particularly, as in the aforementioned turbocharger, vibration of the rotor is promoted by the rotor. The rotation of the rotor thus becomes unstable, producing noise.
Also, if the axial dimension of the end plates is enlarged, the rotor as a whole becomes relatively large in the axial direction. The induction motor also becomes relatively large as a whole. Therefore, it is desirable to suppress enlargement of the rotor in terms of the weight and axial dimension, which leads to various problems.